The invention concerns a closable extrusion nozzle for thermoplastic materials with a nozzle plate containing nozzles that permit extrusion of the plastic material from a hopper, and with a perforated plate arranged upstream of the nozzle plate, whose apertures can be brought in and out of alignment with the nozzles by displacing the perforated plate relative to the nozzle plate.
Such an extrusion nozzle is known from the European Patent 0 093 358. There, the design described in the patent consists of the perforated plate that is located downstream of a flat nozzle plate. The perforated plate is pressed against the nozzle plate by recesses in the nozzle plate and a spring assembly resulting in a seal between the nozzle plate and the perforated plate when the nozzles are not aligned with the apertures of the perforated plate. The nozzles and the apertures are brought in and out of alignment by a sliding mechanism that allows to open and close the extrusion nozzle. The recesses of the nozzle plate grip the rims of the perforated plate like U-channels that result in protrusions on the exit side of the perforated plate that interfere with the access to the entire width and length of the perforated plate. The patent also mentions that the perforated plate can be located above the nozzle plate, but it does not provide any details.